Particulate thermosetting adhesive compositions

ABSTRACT

Particulate thermosetting adhesive compositions exhibiting heat softening properties and capable of resisting penetration into porous substrates can be made at significantly lower cost than conventional adhesives by coating inorganic carrier particles with a previously prepared liquid thermosetting resin and spray drying the resulting mixture.

BACKGROUND OF THE INVENTION

1. Field Of The Invention

This invention relates to particulate thermosetting adhesives andmethods for their preparation. More specifically, the invention relatesto methods for preparing solid particles comprising an inner carriersubstance and an outer coating of thermosetting resin.

2. Description Of The Prior Art

Particulate thermosetting adhesives in the form of finely dividedpowders are extensively used as binders in the manufacture of manyindustrial composites. Powdered phenolic adhesives are used in themanufacture of composite wood products such as waferboard,particleboard, fiberboard and the like. Powdered melamine adhesives areused in the manufacture of molded wood products and molding compounds. Adescription of the processes involved in the manufacture of suchcomposites can be found in "Phenolic Resins" by Andre Knop and Louis A.Pilato, Springer Verlang Berlin Heidelberg, 1985.

A common factor in all these processes is the application of sufficientheat to the composite to allow the thermosetting adhesive powdercontained therein to melt, thereby wetting the substrate surface andsubsequently solidifying to an infusible mass bonding the wettedsurfaces together.

Thus, thermosetting adhesives suitable for the manufacture of industrialcomposites must exhibit heat softening properties to effectively anduniformly coat and wet the substrate surface. However, in themanufacture of composites containing porous substrates such as wood orcotton based materials, adhesives with heat softening properties willnot only coat and wet the substrate surfaces, but will also penetrateand absorb through these surfaces. This effectively reduces the amountof adhesive available at the substrate interface, thereby weakening thestrength of the composite. While an increase in the amount of adhesiveused in the composite would compensate for this loss, such measures arewasteful and result in increased costs.

U.S. Pat. No. 4,424,300 to Udvardy et al discloses a method of making aspray dried phenolic resole resin having heat softening and fast cureproperties. The heat softening properties are imparted by incorporatingin a separate step a novolac resin into a resole resin prior to spraydrying. While this method provides for improved heat softeningproperties of the adhesive, it also results in increased penetration ofthe adhesive into porous substrates and consequently reduced strength ofthe composite. Additionally, the manufacture in separate steps of anovolac and a resole, with subsequent blending of the two is timeconsuming and wasteful.

U.S. Pat. No. 4,098,770 to Berchem et al discloses a method of making aspray dried phenolic resin with a non-phenolic polyhydroxy compoundadded thereto prior to spray drying. The non-phenolic polyhydroxycompound significantly improves the heat softening properties of theresin, but also results in increased penetration of the resin in thecase of porous substrates. Additionally, because of the humectant natureof such non-phenolic polyhydroxy compounds, spray drying of resinscontaining such compounds is difficult and wasteful.

U.S. Pat. No. 3,931,070 to Bond et al discloses a phenol-formaldehyderesin plywood adhesive containing at least 1% of a sodium lignosulfateand up to 1% of a trialkyl phosphate. The addition of trialkyl phosphateand lignosulfate to the phenol-formaldehyde adhesive containingextenders and fillers, especially amylaceous and proteinaceous materialsmodifies the surface properties of the phenolic adhesive mixture toenhance the curtainability or film forming properties to provide aneven, uniform coating of the adhesive on a veneer.

A significant amount of prior art also relates to the use of lignins orwaste sulfite liquor as a binder additive for cellulosic material, suchas, U.S. Pat. No. 4,357,454 to Holmberg et al, U.S. Pat. No. 4,324,747to Sudan et al, U.S. Pat. No. 4,303,562 to Hollis, Jr. et al, U.S. Pat.No. 4,127,544 to Allan, U.S. Pat. No. 4,105,606 to Forss et al, U.S.Pat. No. 3,886,101 to Felicetta et al, and U.S. Pat. No. 3,864,291 toEnkvist.

SUMMARY OF THE INVENTION

Particulate thermosetting adhesive compositions exhibiting heatsoftening properties and capable of resisting penetration into poroussubstrates can be made at significantly lower cost than conventionaladhesives by coating inorganic carrier particles with a previouslyprepared liquid thermosetting resin and spray drying the resultingmixture.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, the particulate thermosettingadhesive composition comprises an aqueous mixture of a thermosettingresin and substantially water insoluble inorganic particulate matterhaving at least 50% by weight of the particles greater than 5 microns,and preferably greater than 10 microns, in the maximum particledimension and at least 50% by weight of particles smaller than 100microns, and preferably smaller than 70 microns, in the maximum particledimension. The amount of water insoluble particulate matter varies fromabout 1 to 240 parts by weight, preferably about 5 to 100 parts byweight, to 100 parts by weight of the solids content of thethermosetting resin.

The liquid thermosetting resin is placed in a vessel under agitation.The particulate matter is added to the vessel and agitation is continueduntil the mixture is uniform and homogeneous. The homogeneous, aqueousmixture of particulate matter and resin is then spray dried to form thedry resin coated particulate composition.

It has also been found that an aminofunctional coupling agent, such asan organosilane or organotitanate added to the aqueous particulatethermosetting medium before spray drying the mixture, further improvesthe properties. The coupling agent improves the bonding between theresin and particulate matter. Organotitanates are disclosed in U.S. Pat.No. 4,069,192 to Monte et al, and organosilanes are disclosed in Katz,"Handbook of Fillers for Plastics" pp 69-70 (Van Nostrand Reinhold Co.,N.Y. 1987), which are incorporated by reference herein. Preferredcoupling agent include aminopropyltriethoxysilane andisopropyltri(N-diethylamino)titanate. The amount of coupling agent canvary from about 0.001 to 1%, preferably about 0.01 to 0.5% by weight ofthe particulate matter.

In the inventive thermosetting adhesive composition the inorganicparticulate matter is essentially water insoluble and inert to thethermosetting resin. The inorganic particulate matter after spray dryingserves as a carrier substance with an outer coating of essentially drythermosetting resin. The combined material exhibits heat softeningproperties and resists penetration into porous substrates such as woodor cotton based materials for which it is used as a bonding agent.

Any water based, liquid thermosetting resin with a solids content of atleast about 10% by weight and not exceeding about 80% by weight andpreferably about 20% to 60% is suitable for this invention. Preferredresins include aldehyde condensation products with phenol, resorcinol,melamine, urea, acetone, dicyandiamide, as well as mixtures andcopolymers thereof. Especially preferred are aqueous phenol-formaldehyderesole resins disclosed in Canadian Patent No. 1,160,389 and U.S. Pat.No. 4,098,770 to Berchem et al, U.S. Pat. No. 4,537,941 to Kambanis etal and U.S. Pat. No.4,324,747 to Sudan et al, which are incorporated byreference herein.

The inorganic carrier substances suitable as the particulate carrier forthis invention are discrete particles of inorganic compositions ormixtures comprised of at least 50% by weight of particles greater than 5microns, in maximum particle dimension, and at least 50% by weight ofparticles smaller than 100 microns in maximum particle dimension, andexhibiting a solubility in water of not more than 0.1% by weight.Preferred particles have minimum particle dimensions of at least 20microns to a maximum of 50 microns. While the physical shape of suchparticles can vary, it has been found that those having a platy orspheroidal shape are particularly suitable for this invention.

Specific inorganic particles useful for this invention include inorganicoxides such as alumina and gibbsite; inorganic salts such as barites,aragonite, calcite, dolomite; and inorganic silicates such as silicatesof zirconium, calcium, calcium-magnesium, alumina, magnesium,sodium-potassium-alumino, as well as hydrous silica, silica and glass;and mixtures such as flyash. Especially preferred are silica, glass andMuscovite mica.

The coupling agent to improve the adhesion of the thermosetting resin tothe inorganic particulate carrier can also be incorporated into theliquid mixture either by adding the coupling agent to the mixture priorto spray drying, or by separately treating the particulate matter withthe coupling agent prior to combining the particulate matter with theliquid thermosetting resin.

The process of treating particulate matter with coupling agents isdescribed in detail in Katz et al, "Handbook of Fillers for Plastics"pages 69-70 (Van Nostrand Reinhold Co., New York, 1987).

Spray drying the thus prepared aqueous particulate containingthermosetting adhesive composition can be accomplished in any suitabledevice, such as in a drier equipped with a spinning disc atomizer. Inlettemperatures in the spray drier can vary from about 180° to 250° C., andoutlet temperatures can vary from about 90° to 120° C. The technique ofspray drying is well known and is described in detail in Perry,"Chemical Engineers' Handbook," pages 20-51 (6th edition, McGraw Hill &Co., 1984), which is incorporated by reference herein.

The spray dried adhesive compositions thus prepared are free flowingfinely divided particles of the inorganic particulate matter having anouter coating of the thermosetting resin, and are useful as binders inthe manufacture of Composites containing porous substrates. The averageparticle size distribution of the composition has 85 weight percentsmaller than 74 microns, with a bulk density varying from about 0.2 to0.4 grams per cubic centimeter.

The spray dried particulate thermosetting adhesive composition can beused in the manufacture of, for example, waferboard, particleboard,fibermat insulation boards and molded wood products. In general, thespray-dried resin is dry blended with the substrate particles, followedby the application of sufficient heat to allow the thermosettingadhesive powder contained therein to melt, thereby wetting the substratesurface and subsequently solidifying to an infusible mass bonding thewetted surface together. In addition to being a useful binder in themanufacture of composites, the particulate or powdered adhesivecomposition of this invention offers other important advantages overconventional powdered thermosetting adhesives, namely, lowermanufacturing costs, improved drying rates and resistance to penetrationof porous substrates.

The amount of particulate adhesive composition will generally varydepending upon the particular wood substrate manufactured. For example,with waferboard, the amount of adhesive composition can vary from about1 to 4% by weight; with fibermat, it can vary from about 10 to 30% byweight; with molded wood products, it can vary from about 10 to 40% byweight; and with particle board, it can vary from about 5 to 1% byweight.

In the forest products industry, adhesive penetration is assessed on thebasis of visual inspection of the glueline. In the case of waferboard, acommon procedure is as follows:

1. Carefully delaminate a portion of the glueline by lifting off waferswith the help of a sharp knife. 2. Expose the delaminated piece to heat,sufficient to result in a brownish discoloration of the resin. A fewminutes in a toaster oven usually suffices, thereby allowing the resinto be visually distinguishable from the wood. 3. Examine the pieces in amicroscope for the following characteristics:

a. Distinct resin particles visible--no penetration; precure.

b. No resin particles visible, resin molten along woodfibers--satisfactory

c. No resin visible--excessive penetration

Since the cost of conventional thermosetting resin adhesives issignificantly higher than many inorganic particulate substances, theincorporation of inorganic particulate substances into a thermosettingresin adhesive, in accordance with the present invention, results incorrespondingly reduced cost of the adhesive without adversely affectingits performance. It has been found that the actual amount of resin usedcan be reduced in an amount proportional to the amount of particulatematter contained in the inventive composition.

Furthermore, in the process of converting the liquid mixture into a drypowder by spray drying, the cost of drying which is related to theamount of water that has to be removed from the mixture, is alsoreduced. Thus, incorporating inorganic particulate matter into themixture in accordance with the invention, reduces the amount of waterthat must be removed from the mixture per unit weight of adhesive,resulting in additional cost savings.

In the examples which follow, the following test procedures were used toarrive at the disclosed values:

(a) Heat Softening Value

A pressed pellet of resin weighing 0.7 grams is placed onto a preheatedhot plate at 140° C. Immediately upon placement, the pellet is coveredby a 10 sq. inch glass plate centered on the pellet. A 14 pound weightis then placed upon the glass plate so that its force is perpendicularto the surface of the glass plate. Three minutes after placement of theweight, it is removed and the resin mass is examined. The heat softeningvalue is determined as the diameter of the circular mass of resinremaining under the glass plate, expressed in millimeters.

(b) Hot Plate Cure

A mass of resin weighing 0.5 grams is placed onto a preheated hot plateat 140° C. The mass is evenly spread over a 20 square centimeter area,with a spatula, and a forceful side to side motion is applied to themass until such time that the solidification of the resin has progressedto the point where it can no longer be moved with the spatula. The hotplate cure is determined as the time elapsed between placement of theresin onto the hot plate and solidification, expressed in seconds.

(c) Thickness Swell and Moisture Absorption

Specimens measuring three inches by ten inches are submerged in water at21° C. for two hours. They are then removed and the excess water isdried off with a towel. The initial weight and thickness of thespecimens are determined, followed by reimmersion of the specimens inwater at 21° C. for 22 hours. The specimens are then removed, excesswater dried off with a towel, and final weight and thickness determined.Thickness swell and moisture absorption are expressed as percentageincrease in thickness and weight respectively.

(d) Internal Bond

The procedure used was in conformance to the Canadian StandardsAssociation (CSA) Standard Test Methods for Mat-Formed WoodParticleboards and Waferboard CAN3-0188.0-M78, par. 6.7, pages 19-20(1978)

(e) Viscosity

Viscosity was measured by Gardner-Holt comparator tubes and expressed inletters.

(f) Solid Content

One gram of resin is deposited onto a glass dish and placed into an ovenat 105° C. for 3 hours. The dish is then allowed to cool to 25° C. in adesiccator and the weight of the remaining resin residue is determined.The solid content is the amount of residue compared to the originalweight of resin sample, expressed as a percentage.

EXAMPLE 1 (a) Preparation of liquid thermosetting resin A:

To 28.35 grams of a 45% formaldehyde solution were added 23.25 grams ofphenol and 17.03 grams of water. The mixture was then heated to 40° C.and 1.88 grams of 50% caustic soda solution were added under agitation.The mixture was heated to 65° C. and held at that temperature for 90minutes. The mixture was then heated to 85° C. and held at thattemperature for 80 minutes. The mixture was then cooled by vacuumrefluxing to 75° C. and maintained at this temperature until the mixtureattained a Gardner-Holt viscosity of N. The mixture was then cooled overa period of 1 to 2 hours to 30° C., followed by the addition of 5.08grams of hexamethylene tetramine, 1.55 grams of 50% caustic sodasolution, 22.45 grams of water and 0.40 grams ofisopropylaminedodecylbenzenesulfonic acid. The resin thus produced had aGardner-Holt viscosity of B and a solids content of 38%.

(b) Preparation of powdered thermosetting adhesive A:

To 100 grams of liquid thermosetting resin A were added 16.28 grams ofMuscovite mica having 100% by weight of particles less than 100 micronsin maximum particle dimension and 80% by weight of particles greaterthan 10 microns in maximum particle dimension, and 0.055 grams ofaminopropyltriethoxysilane. The resulting mixture was spray dried in adisc atomizing drier, yielding a free flowing powder with a heatsoftening value of 45 millimeters, a bulk density of 0.25 grams percubic centimeter, and hot plate cure of 42 seconds.

(c) Preparation of powdered thermosetting adhesive B:

To 100 grams of liquid thermosetting resin A were added 25.3 grams ofglass beads having a diameter of 44 microns, (Potter Industries, LaPrairie, Quebec, Canada) and 0.063 grams of aminopropyltriethoxysilane.The resulting mixture was spray dried in a disc atomizing drier,yielding a free flowing powder with heat softening value of 42millimeters and a hot plate cure of 38 seconds.

(d) Preparation of powdered thermosetting adhesive C:

To 100 grams of liquid thermosetting resin A were added 9.5 grams ofground silica having 99.7% by weight of particles less than 100 micronsand having 70% by weight of particles greater than 10 microns in maximumparticle dimension, and 0.048 grams of aminopropyltriethoxysilane. Theresulting mixture was spray dried in a disc atomizing drier, yielding afree flowing powder with a heat softening value of 46 millimeters and ahot plate cure of 44 seconds.

(e) Preparation of powdered thermosetting adhesive D (control--notcontaining particulate matter):

Liquid thermosetting resin A was spray dried in a disc atomizing drier,yielding a free flowing powder with heat softening value of 48millimeters and a hot plate cure of 46 seconds.

EXAMPLE 2 (a) Preparation of liquid thermosetting resin E:

To 2410 grams of a 45% formaldehyde solution were added 60 grams ofmethanol and 3.7 grams of 25% caustic soda solution. The mixture washeated to 35° C. and 2170 grams of melamine were added. The mixture wasthen heated to 93° C. and held at that temperature to a Gardner-Holtviscosity of A. 360 grams of water were then added, and the mixture wascooled to 30° C. yielding a resin with a solids content of 51%.

(b) Preparation of powdered thermosetting adhesive F:

To 100 grams of liquid thermosetting resin E were added 12.75 grams ofglass beads having a diameter of 44 microns (Potter Industries, LaPrairie, Quebec, Canada), 0.063 grams of aminopropyltriethoxysilane and12.5 grams of water. The resulting mixture was spray dried in a discatomizing drier, yielding a free flowing powder with heat softeningvalue of 78 millimeters and a hot plate cure of 390 seconds.

(c) Preparation of powdered thermosetting adhesive G:

To 100 grams of liquid thermosetting resin E were added 16.7 grams ofMuscovite mica having 100% by weight of particles less than 100 micronsin maximum particle dimension and 80% by weight of particles greaterthan 10 microns in maximum particle dimension, 0.067 grams ofaminopropyltriethoxysilane and 17.0 grams of water. The resultingmixture was spray dried in a disc atomizing drier yielding a freeflowing powder with a heat softening value of 67 millimeters and a hotplate cure of 320 seconds.

(d) Preparation of powdered thermosetting adhesive H (control--notcontaining particulate matter):

Liquid thermosetting resin E was spray dried in a disc atomizing drier,yielding a free flowing powder with a heat softening value of 81millimeters and hot plate cure of 430 seconds.

EXAMPLE 3

Aspen wood flakes were sprayed with 1.5% molten slack wax and mixed with2.5% of powdered thermosetting adhesive. Composites were made bypressing these flakes at 210° C. for 5 minutes to yield boards 7/16 inchthick with a density of 40 pounds per cubic foot. Powdered thermosettingadhesives A, B, C, and D were evaluated in this manner and the followingresults were obtained:

    ______________________________________                                                 Adhesive Internal   Thickness                                                                             Moisture                                 Board No.                                                                              Bond     Bond, psi  Swell % Abs. %                                   ______________________________________                                        1        A        56         16      40                                       2        B        69         14      28                                       3        C        62         17      38                                       4        D        58         18      42                                       ______________________________________                                    

EXAMPLE 4

Composites were made by dry blending 60 grams of softwood flour with 22grams of hardwood flour and 18 grams of thermosetting adhesive resin andpressing this dry blend at 165° C. for 7.5 minutes to yield boards 1/2inch thick and having a density of 60 pounds per cubic foot. Powderedthermosetting adhesives F, G, and H were evaluated in this manner andthe following results were obtained:

    ______________________________________                                        Board No.    Adhesive Interna1 Bond, psi                                      ______________________________________                                        1            F        270                                                     2            G        310                                                     3            H        290                                                     ______________________________________                                    

EXAMPLE 5

Aluminum block composites were made by placing one gram of resin onto a1 inch by 1 inch surface of a 1/2 inch thick block of aluminum. A secondblock of aluminum, identical to the first, was then placed on top andthe assembly was pressed at 140° C. for 4 minutes. Thermosettingpowdered adhesives A, B, C and D were evaluated in this manner and thefollowing results were obtained:

    ______________________________________                                        Composite No.  Adhesive Internal Bond, psi                                    ______________________________________                                        1              A        850                                                   2              B        910                                                   3              C        820                                                   4              D        812                                                   ______________________________________                                    

The particulate thermosetting resins of the present invention producedan internal bond on a non-porous substrate that surprisingly resulted inan increase in strength. Other suitable non-porous substrates of ametallic nature that can be used include brass, copper, steel, chromium,lead, silver, zinc, gold, platinum, magnesium, titanium, and the like.Suitable non-porous plastic materials that can be used include moldedresin products of melamines, phenolics, polyesters and the like.

What is claimed is:
 1. A particulate thermosetting adhesive compositionhaving heat softening properties and the ability to resist penetrationin a porous substrate, comprising individual free flowing finely divideddiscrete particles of substantially water insoluble inorganicparticulate matter inert to and coated with a thermosetting resincomprising at least one aldehyde condensation product selected from thegroup consisting of phenol, resorcinol, melamine, urea, acetone,dicyandiamide, and mixtures thereof; and wherein the amount ofparticulate matter varies from about 1 to 240 parts by weight per 100parts by weight of the solids content of the thermosetting resin.
 2. Thecomposition of claim 1, wherein the proportion of particulate mattervaries from about 5 to 100 parts by weight.
 3. The composition of claim1, also including an aminofunctional coupling agent selected from thegroup consisting of organosilanes, organotitanates and mixtures thereof,wherein the coupling agent varies from about 0.001 to 1.0% by weight ofthe particulate matter.
 4. The composition of claim 3, wherein thecoupling agent varies from about 0.01 to 0.5% by weight of theparticulate matter.
 5. The composition of claim 3, wherein the couplingagent is selected from the group consisting ofaminopropyltriethoxysilane, isopropyltri(N-diethylamino)titanate andmixtures thereof.
 6. The composition of claim 1, wherein the solidscontent varies from about 20 to 60% by weight.
 7. The composition ofclaim 1, wherein the particulate matter comprises discrete particles ofinorganic compositions having at least 50% by weight of particlesgreater than 5 microns in the maximum particle dimension and asolubility in water not more than 0.1% by weight, and are selected fromthe group consisting of inorganic oxides, inorganic salts, inorganicsilicates, and mixtures thereof.
 8. The composition of claim 1, whereinthe free flowing, finely divided individual discrete particles coatedwith the thermosetting resin are obtained by spray drying an aqueousmixture of the particulate matter and resin to form the dry resin coatedparticulate composition.
 9. The composition of claim 1, wherein thethermosetting resin is water soluble.
 10. The composition of claim 1,wherein at least 50% by weight of the water insoluble inorganicparticulate matter comprises particles greater than 5 microns and atleast 50 weight % comprises particles smaller than 100 microns, in themaximum particle dimension.
 11. The composition of claim 1, wherein thethermosetting resin initially is in a liquid state and has a solidscontent of about 10 to 80% by weight.